1. Field of the Invention
The present invention relates to methods for hardfacing various types of metal objects such as tools, bearing surfaces, seals, valve seats, turbine blades, nozzles and the like. In particular, the invention is directed toward a method for applying a hardface metal alloy coating of the type wherein a nitriding step is used to enhance the hardfacing and corrosion resistant properties of the hardfacing metal or alloy.
2. Description of Related Art
Various types of hardfacing operations are known in the art. Typically these operations utilize a hardfacing alloy as an overlay on a structural material where increased resistance to wear is desired. The methods of applying the hardfacing alloy are well known and include welding, such as oxyfuel gas welding, and arc welding; spraying such as plasma spraying; brazing of preformed pieces and laser procedures such as laser alloying, laser cladding and laser melt/particle injection procedures.
A wide variety of hardfacing materials are currently available to suit the particular needs of articles which are subjected to abrasive and corrosive environments. Conventional hardfacing materials include metal carbides, cobalt-based alloys, nickel based alloys, iron based alloys and copper based alloys.
The abrasion and corrosion resistant properties of some hardfacing materials is improved by nitriding the hardfacing material after it has been applied to the substrate metal. For example, U.S. Pat. No. 3,827,920 discloses a method for forming a wear-resistant surface on a metal article which comprises coating a self-fluxing alloy onto the surface of a base metal and subjecting the coating to a nitriding treatment. Likewise, U.S. Pat. No. 3,829,260 discloses an improved wear-resistant metal object which is formed by coating a base metal with an alloy and then, if desired, nitriding the coated article to form a nitrided hardened layer on the object. Similarly, U.S. Pat. No. 3,801,381 discloses alloys which can be coated onto a base metal substrate and then subjected to a nitriding step.
All of the prior art hardfacing procedures which involve a nitriding step, require that the process be carried out in two steps; the first step being the deposition of the hardfacing material and the second step being the subsequent nitriding of the deposited hardfacing material. Consequently, the two step procedure requires more costly equipment and a greater expenditure of time, effort and money than would be required if the entire hardfacing deposition and nitriding procedure could be carried out in a single process step. However, no one has developed any process wherein the hardfacing deposition and nitriding can be carried out concurrently in a single process. Thus, a long felt need in the art exists to consolidate these two process steps into a single operation so as to avoid the disadvantages associated with separate hardfacing deposition and nitriding procedures.